1. Field of the Invention
The present invention relates to a backlight module, and more particularly, to a backlight module capable of utilizing a holder to maintain a gap between a light emitting diode light bar and a light incident side of a light guide plate.
2. Description of the Prior Art
In a side-light type backlight module, in order to fix and position optical components, such as a light guide plate (LGP), on a back plate, a mechanism used for fixing the optical components is often designed on the back plate whereon the optical components and a light source are disposed. The light source can be a light emitting diode module or a cold cathode tube. For example, Taiwan patent No. I266933 discloses an improvement of utilizing a plurality of salient blocks of the optical components to engage with a positioning component of the back plate for fixing and positioning the optical components. A positioning groove can be formed between adjacent salient blocks. The positioning component includes an inner wall and two side walls at least. A light source of the backlight module, such as a light emitting diode, can be disposed on the back plate. The mechanism used for fixing the optical components on the back plate is often disposed on a center of a lateral side of the optical component so as to achieve a preferred effect for fixing and positioning.
However, the light emitting diode of the backlight module is often disposed on a light incident side of a light guide plate, with a gap between the light emitting diode and the light incident side of the light guide plate. In order to increase efficiency of the light emitting diode, the gap should be reduced as possible as it could. It means that the gap between the light emitting diode and the light incident side of the light guide plate influences luminance and uniformity of the backlight module. While the light emitting diode and the light incident side of the light guide plate are entirely closed, that is the gap approaches to zero, the light emitting diode has the best incident efficiency. The incident efficiency decreases while the gap is increased. For example, if the gap is increased from 0 mm to 1 mm, luminance of the backlight module is reduced to 90%. However, it is quite difficult to reduce the gap between the light emitting diode and the light incident side of the light guide plate to 0 mm in mass production. Therefore, the gap between the light emitting diode and the light incident side of the light guide plate is usually controlled within 0.3 mm.
Please refer to FIG. 1. FIG. 1 is a cross-sectional diagram of a light emitting diode light bar 2 disposed on a lateral side of a light incident side I0 of a light guide plate 1 of a side-light type backlight module in the prior art. The light guide plate 1 and the light emitting diode light bar 2 are both disposed on a back plate 3. In a testing process of environmental temperature variation of the backlight module, the light guide plate 1 suffers the temperature variation from 0° C. to 60° C. In a transport process of the backlight module, the light guide plate 1 suffers the temperature variation from 0° C. to 50° C. Linear thermal expansion coefficient of the ordinary light guide plate 1 is around 83/(106K). For example, length of a lateral side S0 of a 20 inch display panel is around 445 mm. If the 20 inch display panel suffers the temperature variation of 25 degrees in centigrade, length of the lateral side S0 of the light guide plate 1 expands to 445.92 mm. However, for the conventional backlight module, there is no mechanism capable of fixing a gap H0 between the light incident side I0 of the light guide plate 1 and the light emitting diode light bar 2 in a direction whereto the light guide plate 1 expands. Therefore, the light emitting diode light bar 2 is pressed by expansion of the light guide plate 1 so as to withdraw the light emitting diode light bar 2 from an original position. After the temperature variation of the light guide plate 1 is reduced, the light emitting diode light bar 2 can not be moved back to the original position due to deformation of the mechanism used for fixing the light emitting diode light bar 2 or the light guide plate 1. The gap H0 between the light emitting diode light bar 2 and the light incident side I0 of the light guide plate 1 is increased to a gap H0′. Difference between H0′ and H0 might reach to 0.92 mm at most so as to reduce the incident efficiency of the light emitting diode light bar 2 emitting light to the light incident side I0 of the light guide plate 1, and luminance of the backlight module is reduced to 90%.